A FIELD-STUDY ON THE FATE OF N-15-AMMONIUM TO DEMONSTRATE NITRIFICATION OF ATMOSPHERIC AMMONIUM IN AN ACID FOREST SOIL

To demonstrate the contribution of atmospheric ammonium to soil acidification in acid forest soils, a field study with N-15-ammonium as tracer was performed in an oak-birch forest soil. Monitoring and analysis of soil solutions from various depths on the N-15-ammonium and N-15-nitrate contents, showed that about 54% of the applied N-15-ammonium was oxidized to nitrate in the forest floor. Over a period of one year about 20% of the N-15 remained as organic nitrogen in this layer. The percentage N-15 enrichment in ammonium and nitrate were in the same range in all the forest floor percolates, indicating that even in extremely acid forest soils (pH < 4) nitrate formation from ammonium can occur. Clearly, atmospheric ammonium can contribute to soil acidification even at low soil pH.